The purpose of the proposed research is to identify the mechanism by which the lymphocytosis-promoting factor (LPF) from B. pertussis exerts its effect, not only in causing lymphocytosis but also with respect to its other activities as histamine-sensitizing factor and as a potent mitogen for murine T-cells which also results in the generation of non-specific cytotoxic lymphocytes. It has been found that T cell activation requires helper B cells and that the reaction can be blunted by suppressor T cells. The nature of this three-cell interaction is being explored. In addition, sub-units of LPF are being isolated and tested for activity.